The privilege was removed because of abuse. Unfair usage as in users typically closing own threads for personal reasons rather than with regard to the topic and the posting community, such marked by the confusion and dissent created--and also that so many users with the ability means so much room for mischief. Spam, of course, being the most notable: this was made more evident when the sorting system changed for several months, where spam posting could be performed, locked up, and conveniently tucked away. That the defunct checkbox remained for so long was simply a design oversight. Logistics and withholding seem to be decent grounds for the change.
Limxzero's posts always amazes me because he beats me to these kind of things. It also did prevent spam with pointless threads which I'm thankful that it's finally removed.